It begins in a Stockholm city park where the abused body of a young boy is discovered. Detective Superintendent Jeanette Kihlberg heads the investigation, battling an apathetic prosecutor and a bureaucratic police force unwilling to devote resources to solving the murder of an immigrant child. But with the discovery of the mutilated corpses of two more children, it becomes clear that a serial killer is at large.

Superintendent Kihlberg turns to therapist Sofia Zetterlund for her expertise in the psychopathology of those who kill, and the lives of the two women become quickly intertwined—professionally and personally. As they draw closer to each other and to the truth about the killings, what surfaces is the undeniable fact that these murders are only the most obvious evidence of an insidious evil woven deep into Swedish society.